1. Field of the Invention
The present invention relates in general to a vehicle having a set of steerable front wheels (for main steering) and a set of steerable rear wheels (for secondary steering) and more specifically to a steering control system for controlling the steering angles of the front and rear wheels based on the vehicle running conditions in order to improve the vehicle running characteristics such as the straight-ahead running characteristic, the turning characteristic, etc.
2Description of the Prior Art
An example of a prior art steering control system of this kind is disclosed in Japanese Provisional Patent Publication No. 61-115776. In the prior art steering control system, a desired vehicle behavior or motion characteristic (e.g., a desired response-to-yaw rate characteristic) is set as a reference model so that a rear wheel steering angle capable of attaining a yaw rate determined by the reference model is calculated based on the vehicle dynamics.
The prior art steering control system is provided with a plurality of references models which are selected by a switching or changeover operation so that a plurality of driving modes are available.
However, since a plurality of driving modes are selectively and readily available by a simple switching operation, there arises the problem that if the selected driving mode is changed from one to another during running of the vehicle the steering angle of the rear wheels is changed abruptly, causing an abrupt vehicle behavior that is not only uncomfortable but is undesirable from a safe driving point of view.
In order to solve the above problem, it is necessary to construct the system so that the above switching or changeover can be effected only when the rear wheel steering angle is zero or nearly zero. However, even with this structure, the above described problem cannot be solved completely and to a satisfactory extent.